In the following, the terminologies “connected” and “connection” is intended to mean “optically connected” and “optical connection”, respectively.
FIG. 1 is a block diagram illustrating a passive optical network WDM-TDM PON. A passive optical network PON has a point-to-multipoint network architecture. It comprises an optical line terminal OLT 20 connected to a plurality of optical network units ONU 30 through a plurality of optical splitters or multiplexers 31 and optical fiber portions 22, 32. Typically, the optical line terminal 20 comprises a receiver 23 and a transmitter 24 located at a central office of a service provider, and serves the plurality of optical network units 30 located near end users. The receiver 23 and the transmitter 24 are connected to the optical fiber portion 22 through a circulator 21. The transmitter 24 of the optical line terminal 20 transmits optical signals towards the optical network units 30. The receiver 23 of the optical line terminal 20 receives optical signals sent by the optical network units 30. These downstream and upstream signals using different wavelength bands share the same optical fiber portions 22, 32. Generally, due to the attenuation of the optical signals travelling in long optical fiber portions, the passive optical network PON further comprises a bidirectional optical amplifier 1.
FIG. 2 is a block diagram illustrating the bidirectional optical amplifier 1. The bidirectional optical amplifier 1 is passed through in one direction by a downstream optical signal SDS and in an opposite direction by an upstream optical signal SUS. It comprises a first optical circulator 2 and a second optical circulator 3, both having three ports P1, P2, P3. A first port P1 of the first optical circulator 2 defines a first connector 8 at one end of the bidirectional optical amplifier. A first port P1 of the second optical circulator 3 defines a second connector 9 at an opposite end of the bidirectional optical amplifier. The first connector 8 is connected to an optical fiber portion 32. The second connector 9 is connected to another optical fiber portion 22. A downstream amplification path 5 for the downstream optical signal SDS is defined between a second port P2 of the first optical circulator 2 and a second port P2 of the second optical circulator 3. A downstream unidirectional optical amplifier 4 is connected between said ports in the downstream amplification path 5. An upstream amplification path 7 for the upstream optical signal SUS is defined between a third port P3 of the first optical circulator 2 and a third port P3 of the second optical circulator 3. An upstream unidirectional optical amplifier 6 is connected between said ports in the upstream amplification path 7 for the upstream optical signal. Typically, such a bidirectional optical amplifier comprises rare earth doped fiber amplifiers DFA as unidirectional optical amplifiers 4, 6. A rare earth doped fiber amplifier comprises a rare earth doped optical fiber as a gain medium to amplify the optical signal. As an example, the rare earth doped fiber amplifier may be an Erbium doped fiber amplifier EDFA.
The bidirectional optical amplifier 1 as depicted in FIG. 2 is not satisfactory because of instability caused by the imperfect isolation between the second P2 and third P3 port of each optical circulator. The effects of imperfect isolation on the downstream signal SDS (full line arrows) and the upstream signal SUS (broken line arrows) at the different amplification stages are depicted in small frames. More precisely, a part of the output power 11 of the upstream unidirectional optical amplifier 6 is re-injected at the input of the downstream unidirectional optical amplifier 4. Similarly, a part of the output power 12 of the downstream unidirectional optical amplifier 4 is re-injected at the input of the upstream unidirectional optical amplifier 6. The corresponding optical power re-circulates in a closed loop in the bidirectional optical amplifier 1. As a consequence, such a bidirectional optical amplifier 1 is not stable, namely the output power of the bidirectional optical amplifier in both directions varies erratically over the time.
It has been proposed bidirectional optical amplifiers comprising unidirectional optical amplifiers having low output power, or comprising circulators having increased isolation ratio. However, these solutions do not satisfactorily prevent oscillation of the bidirectional optical amplifier output signal. Further, a circulator having an isolation ratio of 35 dB requires the output power of the unidirectional optical amplifiers to be limited to 16 dBm in order to prevent oscillation. This is not acceptable because it reduces the reach (distance between OLT and ONU) and splitting ratio (number of ONU per OLT) of the passive optical network.